Neocarzinostatin (NCZ), an acidic polypeptide antibiotic, was given to 47 patients with cancer and leukemia, and tolerance to two schedules, a single dose given as a 2 hour infusion and a continuous infusion over 5 days was investigated. Immediate reactions, including fever, chills, rigor, hypertension and mental confusion, were dose-limiting for the 2 hour infusion schedule, occurring at 3000 U/m2 and higher. Continuous administration for 5 days eliminated the immediate reactions and then hematological toxicity--often prolonged leukopenia and thrombocytopenia--became dose-limiting. Other toxicities of NCZ at both dose schedules included anemia, fever and chills, anorexia, nausea and vomiting, hepatic dysfunction, azotemia, hypophosphatemia, aminoaciduria, stomatitis, phlebitis and/or cellulitis at the venous infusion site and pruritus. Patients with solid tumors who had received little or no prior chemotherapy and had good bone marrow reserve tolerated up to 6000 U/m2/24 hours X 5 days. One patient with previously treated acute myelocytic leukemia was induced into a good partial remission lasting 10 weeks.